The present invention relates to techniques for reducing the wake-up time of a light-emitting device, such as a Vertical Cavity Surface Emitting Laser (VCSEL), used in a data communication link.
Typically, in optical and electrical data communication links the utilization is mainly in burst mode. Thus, the link may be idle most of the time. For example, in a typical data center, data communication links may be idle 90% of the time. In order to save power, it would be advantageous to put the link into “sleep mode” during periods where the channel is idle. In an optical link in sleep mode, the transmit circuitry and the laser driver may be partly turned off. In addition, in optical links that use Vertical Cavity Surface Emitting Lasers (VCSELs), the bias current to the VCSEL device may be reduced. The VCSEL is a type of semiconductor laser diode in which the laser beam is emitted perpendicularly from the top surface of the device.
However, a problem arises when the data communication links are taken out of sleep mode. Even if the laser bias is kept above the lasing threshold, the “wake-up” of a VCSEL may require a significant length of time. For example, after a prolonged period of reduced bias or low voltage input data signal (logic zero), when the laser bias is increased or a high voltage data signal (logic one) is applied, the amplitude of the optical signal produced by the VCSEL may increase relatively slowly, and may take as long as 20 ns to 40 ns to reach full amplitude. Typically, the receiver at the other end of the data communication link cannot lock on such a signal, so the data communication link cannot transmit usable data for as long as 20 ns to 40 ns.
Typical data packets are short. For example, typical Ethernet packets may be between 10 ns to 256 ns at 56 GB/s. A 20 ns to 40 ns startup time, plus a 20 ns preamble to synch the receiver may result in an overhead of, for example, 23% to 600%, depending on the packet length.
Accordingly, a need arises for techniques for decreasing the wake-up time of a VCSEL used in a data communication link.